1. Field of the Invention
This invention relates to apparatus and methods for determining the amount of compressed natural gas dispensed from a storage tank under pressure to a receptacle, for example a tank, carried by a vehicle under a pressure lower than the pressure of the storage tank.
2. Description of the Prior Art
It has long been known that gasoline engines of the type used in automotive vehicles can, with relatively minor modifications, be made to run efficiently on compressed natural gas (CNG). It is fairly common practice for fleet owners, such as utilities and, particularly, gas utilities, to convert their vehicle fleet to operate on GNG. However, the systems and methods of measuring the amount of CNG dispensed to the individual vehicles has, in the past, produced relatively imprecise results. This is particularly true of gas utilities, since the utility is using its own product and only imprecise accounting methods are employed, if any at all.
Because of the escalating cost of conventional fuels, such as gasoline and diesel fuel, in recent years and the fact that the products of combustion of CNG do not have as great a polluting effect on the atmosphere as do such conventional fuels, interest in converting vehicles used by the general public to operate on CNG has intensified. However, before such interest can be economically developed, methods and systems for dispensing CNG to the general public which are much more precise and accurate than those employed to data would have to be developed. One obvious approach to measuring the amount of gas dispensed to the consumer would be to meter the product in a manner somewhat similar to that which is used in connection with conventional gasoline. Because the pressures in the storage tank would be high (e.g. 3600 p.s.i.) and the pressures in the vehicles which could be anything from atmospheric pressure to a relatively high pressure (e.g. 2400 p.s.i.), the use of mass meters which directly measure the mass of the dispensed fluid may be considered as one approach. However, the use of such meters would be difficult and expensive under the conditions in which they would have to operate.